The present invention relates to a process for coating the sheared edges of pouring or drinking outlets stamped out in the metal lids for beverage cans.
Metal lids for beverage cans, usually made of steel, aluminum or an aluminum alloy, have to be fitted with a facility by means of which an outlet can be made in the lid to allow the contents to be poured out. The best known facility of this kind is the so called ring-pull-system which provides a notch in the lid surface to delineate the predestined opening. This notched region is then torn apart when opening the can. In newly developed systems the outlet for pouring or drinking is first stamped out of the lacquer coated sheet then closed off by sealing a pull-off tab over the said outlet. The edge around the opening in the lid remains unprotected which, especially in the case of lids made of steel sheet, in unfavorable for two reasons. Firstly, the bare metal is exposed to chemical attack by the contents of the can. Secondly, the contents can take up metal by dissolving the same in it, which is undesireable, especially in the case of heavy metals.
Attempts have therefore been made to protect the bare edge of the opening in the lid in order to prevent corrosive attack of the lid and contamination of the contents by dissolved metal.
It has, however, been found particularly difficult to cover over the burrs of metal at the edge which occur increasingly with increasing wear of the stamping tool. With normal spraying methods it is not possible to achieve full coverage of the edge with a fluid lacquer, this because of the particular characteristics of an edge. Full coverage is not even possible with repeated application of the lacquer.
It has also been attempted to protect the edges of these openings with a thick layer of a PVC plastisol. Use of this method on an industrial scale, however, is not feasible, partly because of the large material consumption involved and partly because of the risk of softener in the plastisol migrating into the contents of the can.
The object of the present invention is therefore to develop a process for coating the sheared edges of openings for pouring or drinking stamped out of metal lids for beverage cans, such that the said process permits complete coverage of the sheared edges, especially the burrs thereon, and employs for that purpose a coating material that is in conformity with foodstuffs. The pore test (WACO) should then register a limiting current of, at most, 0.4 mA at a voltage of 6 V in a 1 wt-% NaCl solution. Further, the process should feature short coating and drying times, a minimum treatment rate of 60 lids per minute, and be realisable with minimum material consumption in a unit at reasonable cost.